Power impact hammer



Jan. 24, 1967 P, D, CABOT ET AL 3,299,957

POWER IMPACT HAMMER Filed July 16, 1964 2 Sheets-Sheet l Jan. 24, 1967 y p, D, CABO-|- ET Al. 3,299,967

POWER IMPACT HAMMER Filed July 16, 1964 2 Sheets-Sheet 2 United States Patent Olice 3,299,967 Patented Jan. 24, 1967 POWER IMPACT HAMMER Philip D. Cabot and Robert D. Lowry, Winchester, Mass.,

assignors to Lowry Development Corporation, Winchester, Mass., a corporation of Massachusetts Filed .Iulv 16, 1964. Ser. No. 383,031 9 Claims. (Cl. 173--116) v This invention relates to power driven hammers and more particularly to a powered nail driving hammer adapted to -drive large nails with greater eiciency than has heretofore been achieved.

It is a particu-lar object of the invention to provide a tool for driving nails, from 12d up to spikes of 40-50d size, as into wood or other construction materials with great rapidity, less power consumption, and greater handling convenience.

Powered impact hammers heretofore known to me operate on the principle of very rapid back-and-forth positive drive of a nail head engaging `driven member, usually from a rotating driving member through a crank. In such `devices the reciprocating driven member thus must move through its complete stroke in each cycle, and usually its forward stroke occupies only half of the cycle. The result is that energy is dissipated into retracting the tool when the impact element encounters undue resistance to the completion of its forward stroke. In addition, the speed of return of the impact member requires high horsepower and hence a hefty motor.

Another kind of tool is shown, for example, in U.S. Patent No. 2,905,034, which utilizes a cam to retract a plunger which is mounted between two compression springs to load the stronger of the two springs, releasing the plunger against a shock absorbing weaker spring. In this case the nail head engaging `surface is part of the plunger and has an equal stroke therewith.

In accordance with this invention, the main drive from the power supply acts solely to store driving energy in a compression spring during a retraction stroke, which energy is released to give a substantially free forward ight as a captive projectile of a primary driven member, but this member is not directly connected to the nail head engaging impact member. Instead, vastly superior nail driving results are procured by allowing the nail head-engaging impact member to have a longer travel than that of the projectile and also fabricating the impact element in such manner that it has greater mass than the primary driven member. By this means greater depth of nail insertion can be accomplished per stroke where resistance to nail insertion is low, without sacrificing maximum high energy impact of the tool when resistance to nail insertion is high and depth of insertion per stroke is low.

To these ends, the projectile transmits its energy in its forward ight to the nail head engaging member pneumatically through the interposition of an air chamber therebetween, which acts, during the driven return stroke of the projectile, as a suction chamber and, on the forward stroke, to decelerate the impact rod during the terminal portion of any forward stroke away from the decelerating or decelerated projectile.

In addition, in the preferred form of the invention, a second air chamber is interposed between an internal head of the impact rod and the forward portion of an enlarged breech in the housing. This air chamber serves additionally to decelerate the impact rod head when the decreasing volume in this second air chamber creates an air pressure exceeding the pressure of the air in the rst air chamber.

These and other objects and features of the invention will be more fully understood when taken in connection with the description of a typical embodiment of the invention as shown in the accompanying drawings, wherein:

FIG. l is an elevational view of a hammer in accordance `with this invention showing the parts in fully eX- tended position with a nail in position ready to be driven into an opposing surface;

FIG. 2 shows the hammer with the parts in their positions upon completion of driving of a nail;

FIG. 3 is an enlarged cross-sectional View taken along the line 3 3 of FIG. l;

FIG. 4 is a longitudinal sectional view of certain of the parts of the hammer shown in FIG. l, broken away to indicate extent;

FIG. 5 is a similar cross-sectional view of a'portion of the parts shown in FIG. 4 but in a different position;

FIG. 6 is a cross-sectional view taken along the line 6-6 of FIG. 4; and

FIG. 7 is a cross-sectional view taken along the line 7-7 of FIG. 4.

The hammer comprises a housnig 1t) into which extends a shaft 11 driven through a gear box 12 by a high speed electric motor 13 mounted on one side of the housing 10. As shown in FIG. 4, the shaft 11 extends across the bore of the housing into an outboard bearing 14 mounted on the other side of the housing after passing through two aligned slots 16 in a hollow rod 18 which carries beyond the shaft 11 a pair of cam followers 20 and 22 interposed in the path of cams 24 and 26 keyed on shaft 11. Above cam followers 20 and 22, the top end of hollow rod 18 passes between four rollers 27 (two only are shown) spaced 90 from one another which engage Iiats on the outside surface of rod 18. Rod 18 terminates at its top end with a fitting 23 which engages, through antifriction roller bearings 30, an annular seat member 32. A strong compression spring 34 is inserted between seat 32 and the top or rear end of the housing 10.

The lower or forward end of hollow rod 18 takes the form of a piston 38 contained within a cylinder head 40 slidably mounted in the bore of housing 10 at its lower or forward end. An annular centrally apertured cap 42 forms an abutment limiting movement of cylinder member 40 downwardly but permits the attachment thereto of an impact rod 50 mounted for reciprocating movement through a bushing 52 contained within the bottom barrel 54, which constitutes an extension of the cap 42.

An internal liner 55 in the housing acts as a cylinder for the head 40, the walls 56 having radially spaced ports 58 and 59 for a purpose to be later described.

Telescopically mounted over the lower end of the barrel 54 is a work-engaging sleeve 6i) which is continuously urged downwardly away from the barrel 54 and housing 10 by the interposition `of a relatively light weight balancing compression spring 62 with a resilient gasket 63 interposed between opposed bushings 64 on member 60 and 65 fast on barrel 54.

Before describing the operation of the hammer, it should be noted that the head 4t of impact rod 50 is not connected to the piston 38, but is driven by the piston 38 through the ai-r cushion provided by the intervening cham! ber 70. A notch in the end of piston 38 insures that air will always be present between the piston end and the head end wall. The presence of non-conforming surfaces providing such an air reservoir is important in preventing unwanted permanent interlock of the piston and cylinder.

It is further to be noted that the bottom or forward end of the head 40 is decelerated on its downward or forward stroke by the air cushion provided in the chamber 72 formed between the bottom or forward end of the head 40 and the internal surface of the cap 42 and surrounding liner walls 56. Air chambers 70 and 72 are present because the distance between the opposed end walls -of head 40 is less than the distance between the inside surface of cap 42 and the forward end of piston 3S when the rod 18 is at the limit of its forward stroke. FIG. 4 also shows the impact rod head 40 at the limit of its forward stroke when it has encountered no external resistance, having overtraveled with respect to the rod 18 against the decelerating influence of the compression of air in the lower forward chamber 72. Because of the overtravel, the impact lrod head 40 has drawn a vacuum in the rear chamber 70 and the position of the parts as shown in FIG. 4 will only be instantaneous because, as soon as the inertia of the head 40 has been overcome, the pressures in chambers 72 and 70 will tend to equalize thereby enlarging the volume of chamber 72 and diminishing the volume of chamber 70. Almost simultaneously the commencement of retraction of the rod 18 by the cams 24 and 26 will suck the head 40 away from the cap 42 and as soon as the lower end of the head 4t) has reached the ports 58, any vacuum being drawn in the lower chamber 72 will be re# lieved since ports 58 communicate through annular channel 74 with ports 59 and thence to the internal bore, and the head 40 is in effect sucked upwardly into the position shown in FIG. 5, leaving cham-ber 70 with a very small volume when the cams 24 and 26 have loaded spring 34 into fully retracted position as shown in FIG. 5.

If the surfaces of piston 38 and the opposed walls of head 40 completely conform, with no provision for an air reservoir as provided by the notch 75, it has been found that, as operation proceeds, air chamber 70 tends to be completely exhausted of air without opportunity for replenishment on each stroke with the result that the two parts begin to move as one interlocked by a permanent vacuum therebetween. This would defeat the overtravel which is an important feature of the invention. The slight non-conformance of the surfaces may be provided in other manners, either by rounding the front piston corners or by undercutting the head 40 near or in its end wall. The volume of the reservoir may be small amounting to only a matter of a fortieth of a cubic inch for an 1%" diameter piston, that being more than would be present with conforming surfaces obtained with normal high quality machine production designed to provide the required retraction vacuum.

The contour of cams 22 and 24 is shown in FIG. 3 illustrating that the cams act to load the spring 34 through preferably about 80%, and at least 67%, of a cycle with the followers contacting the low portions of the cams within a sector of preferably about In this manner, a large percentage of the cycle time-wise, e.g. a 280 and at least a 240 sector, is utilized for loading at constant motor to-rque and a very small portion not greater than 45 for the remaining portion of the cycle, including the firing. This reduces the required motor horsepower, and hence the motor weight, which is important for a portable tool.

Should the forward end of impact rod 50 strike a nail positioned in the sleeve 60 before the head 40 has reached the terminus of its stroke, it will Complete its stroke against the decelerating action of the air cushion in cham- '.ber 72 unless the resistance of the nail is greater than the 'energy contained in the head 40, in which case the head -40 may not have any overtravel but will dissipate its entire energy into a driving force rather than cause the tool to withdraw from the nail as was the case in prior devices.

If the combined mass of head 40 and impact rod 50 is made greater, e.g. 50% more than the mass of rod 18 or even greater, a mechanical advantage is gained. Overall weight of the tool, combined with sufficient strength in rod 18 create a limitation in this respect for a portable tool.

Ina typical hammer for driving 16 d. nails, a combined impact rod and head weight was 31/2 lbs., giving a calcu-V lated energy of V200 Vinch pounds per stroke using an `ap;

propriate springl with a 1/2 inch overtravel and a lls" Cam fall, and a mass ratio of about 2:1, including one-half the weight of the spring. For this operation, gear box 12 was arranged to operate the cams at approximately 468 r.p.m. This permitted driving a 16 d. nail into spruce or r with no more than 12-18 blows, i.e. in less than two seconds. This is important not only for the favorable time factor, but also because it has been determined that, to completely drivea given nail, the total energy transmitted by the tool must increase as the number of blows increases and the energy per blow decreases. However, an appropriate range might be between 300 and 1,000 r.p.m., preferably 40G-500 r.p.m., in cont-rast to commercially available impact hammers which operate in the range of 1,800-2,400 blows per minute.

The sleeve 60 permits a nail to be placed in its bore in alignment with the impact rod 50 and then if the parts are in the position shown in FIG. 1, the weight of the upper assembly supplemented by a light push by the operatoi will cause the barrel to travel down into the sleeve 60 compressing the spring 62 until the bottom of the impact rod 50` nears or touches the nail head. Preferably, the spring 62 almost counterbalances the weight of the upper assembly so that the operator maintains control of the operation by slight downward or forward pressure. When the spring 62 is almost fully compressed as shown in FIG. 2, the bottom end of impact rod 50 extends just slightly beyond the lower end of the guide sleeve 60 to slightly `countersink the nail in the material into which it has been driven.

It is to be understood that the opposed end configurations of piston 38 and head 40 may be reversed or may be modified to act as opposed rams operating within a common internal cylinder wall.

What is claimed is:

1. A lpower impact hammer comprising v a housing having a forward barrel leading into a coaxial enlarged bore,

a work-engaging impact rod mounted for reciprocation in said barrel and having an inner head disposed within said bore,

a captive projectile contained within said bore for re.

ciprocation with respect to said housing behind'said,

head with a maximum stroke of less length than the maximum length of the path of reciprocation of said head,

power means for cyclically propelling said projectile towards said head,

and the adjacent ends of said projectile and said head co-operating to form an intervening always un* vented air chamber causing cyclical inertial overtravel of said head away from said projectile as said projectile decelerates on its forward stroke expanding and lowering the air pressure in said unvented chamber and causing cyclical vacuum transmission of retro action motion of said projectile to said head causing said head to recover its said overtravel as said un* vented chamber contracts during retraction ofl said projectile,

2. A `power impact hammer as claimed in claim 1, wherein the means for cyclically propelling said captive projectile includes a compression spring mounted between said housing and the rear end of said projectile and cooperating cam and cam follower means mounted on said housing and said projectile for cyclically retracting said projectile to load said spring and for releasing said projectile vfor forward ight imparted by unloading of said spring. v

3.l A power impact hammer as claimed in claim 2, wherein said cam means includes a rotatable cam, a lobe on said cam having a leading surface of gradually increasing radius extending over at least a 240 sector and a trailing surface of abruptly decreasing radius extending over a sector not exceeding 45 whereby during revolution of said cam said compression spring is gradually 5 loaded during at least 67% of the cam cycle and thereafter abruptly released.

4. A power impact hammer as claimed in claim 1, having a work-engaging sleeve telescopically mounted over the barrel for limited sliding movement relative thereto, said barrel having a hollow front end extending beyond the front end of said housing, and surrounding the path of travel of said rod, and resilient means urging said sleeve into extended position relative to said housing.

5. A power impact hammer as claimed in claim 1, wherein the combined mass of said rod and head exceeds the mass of said projectile.

6. A power impact hammer as claimed in claim S, wherein the mass of said rod and head is in a ratio to the mass of the projectile of at least 3 to 2.

7. A power impact hammer as claimed in claim 1, wherein the adjacent ends `of said projectile and said head have slightly non-conforming surfaces providing an al ways closed unvented air chamber containing a reservoir of air therebetween when said projectile and head are at the terminus of ttheir approaching motion to prevent vacuum interlocking thereof.

8. A power nail driving hammer comprising a housing having a forward barrel leading into a caxial enlarged bore,

a work-engaging impact rod mounted for reciprocation in said barrel,

a captive projectile contained within said bore for reciprocation With respect to said housing behind said rod with a maximum stroke of less length than the maximum length of the path of reciprocation of said rod,

power means for cyclically propelling said projectile towards said barrel,

the inner end of said rod including a double-ended head disposed within said bore between said captive projectile and the forward end of said bore,

said head having opposed end walls facing front end Walls, respectively, of said bore and of said projectile, the distance between said opposed head walls being less than the distance between said bore and projectile end walls at the limit of said projectiles forward stroke to permit said head to overtravel forwardly away from said projectile,

the adjacent ends of said projectile and head co-operating to form an intervening rear always unvented air chamber for cyclical air-cushion transmission of forward thrust from said projectile to said head as said chamber contracts, inertial overtravel of said head as said chamber re-expands and cyclical vacuum transmission of retractive motion of said projectile to said head to overcome its inertia and cause recovery of its overtravel away from said projectile during the return stroke of said projectile,

the forward ends of said bore and said head co-operating to form a front air chamber in advance of said head providing an air cushion for terminal forward motion of said head.

9. A hammer as claimed in claim 8, having a port for venting said front air chamber as said head retracts to break any vacuum developing in said front chamber during retraction of said head.

References Cited by the Examiner UNITED STATES PATENTS 2,342,661 2/1944 Pyle 173-123 2,890,455 6/ 1959 Etzkorn 173-128 3,051,135 4/1962 Smith 173-169 3,167,135 1/1965 Wilmer 173-31 FOREIGN PATENTS 17,862 7/1910 Great Britain.

FRED C. MATTERN, JR., Primary Examiner.

L. P. KESSLER, Assistant Exam ner. 

1. A POWER IMPACT HAMMER COMPRISING A HOUSING HAVING A FORWARD BARREL LEADING INTO A COAXIAL ENLARGED BORE, A WORK-ENGAGING IMPACT ROD MOUNTED FOR RECIPROCATION IN SAID BARREL AND HAVING AN INNER HEAD DISPOSED WITHIN SAID BORE, A CAPTIVE PROJECTILE CONTAINED WITHIN SAID BORE FOR RECIPROCATION WITH RESPECT TO SAID HOUSING BEHIND SAID HEAD WITH A MAXIMUM STROKE OF LESS LENGTH THAN THE MAXIMUM LENGTH OF THE PATH OF RECIPROCATION OF SAID HEAD, POWER MEANS FOR CYCLICALLY PROPELLING SAID PROJECTILE TOWARDS SAID HEAD, AND THE ADJACENT ENDS OF SAID PROJECTILE AND SAID HEAD CO-OPERATING TO FORM AN INTERVENING ALWAYS UNVENTED AIR CHAMBER CAUSING CYCLICAL INERTIAL OVERTRAVEL OF SAID HEAD AWAY FROM SAID PROJECTILE AS SAID 